I have written some code in VB that verifies that a particular port in the Windows Firewall is open, and opens one otherwise. The code uses references to three COM DLLs. I wrote a WindowsFirewall class, which Imports the primary namespace defined by the DLLs. Within members of the WindowsFirewall class I construct some of the types defined by the DLLs referenced. The following code isn't the entire class, but demonstrates what I am doing.

Imports NetFwTypeLib
Public Class WindowsFirewall
    Public Shared Function IsFirewallEnabled as Boolean
        Dim icfMgr As INetFwMgr
        icfMgr = CType(System.Activator.CreateInstance(Type.GetTypeFromProgID("HNetCfg.FwMgr")), INetFwMgr)

        Dim profile As INetFwProfile
        profile = icfMgr.LocalPolicy.CurrentProfile

        Dim fIsFirewallEnabled as Boolean
        fIsFirewallEnabled = profile.FirewallEnabled

        return fIsFirewallEnabled
    End Function
End Class

I do not reference COM DLLs very often. I have read that unmanaged code may not be cleaned up by the garbage collector and I would like to know how to make sure that I have not introduced any memory leaks. Please tell me (a) if I have introduced a memory leak, and (b) how I may clean it up.

(My theory is that the icfMgr and profile objects do allocate memory that remains unreleased until after the application closes. I am hopeful that setting their references equal to nothing will mark them for garbage collection, since I can find no other way to dispose of them. Neither one implements IDisposable, and neither contains a Finalize method. I suspect they may not even be relevant here, and that both of those methods of releasing memory only apply to .Net types.)